SQL2019 Std Quantity License Keys and multiple cloned backup servers (Not Core related)

Hi All,
I'm in need of knowing how many SQL2019 Standard license key I need in my scenario; I'm hoping someone could help so I'm compliant with Microsoft key usages. I'm not a SQL person, I'm the DB Admin but falls upon me to provide software to others.

My server speck's:
4x HP DL360 G9s, 2x processors, 16cores total, 64Gb HP RAM, WS2019 all identical for redundancy purposes (Primary, secondary, tertiary and a forth).

Scenario:
The primary is always running (PBX server), the other three are dormant unless needed if the primary is down, the secondary would be brought online until repairs are made to the primary, then I would switch back to the Primary. The same goes for the other servers but at NO given time are two servers running at the same time although all are online to keep data in sync.

All servers are a clone of the primary servers so hardware and software are 100% identical, I don't use any VM or clustering of any kind and isn't needed in our setup. All servers SQL data is kept in sync via another software in case we need to failover to another server as our disaster recovery solution.

Note: SQL Standard is being used because upgrading or downgrading SQL on those servers won't be needed or an option for many reasons. I know some will suggest ENT but we won't be using any of the ENT features and far more expensive Unless there's another option?

Question:
Since only one server is live at any given time, and Microsoft allows some redundancy in their rules, do I only need One key for all servers or do will I still need to provide a separate key for each server despite having only One server in live production?